Genetic structure of avian myeloblastosis virus, released from transformed myeloblasts as a defective virus particle

Emerging role of MYB transcription factors in cancer drug resistance

Decades ago, the viral myeloblastosis oncogene v-myb was identified as a gene responsible for the development of avian leukemia. However, the relevance of MYB proteins for human cancer diseases, in particular for solid tumors, remained basically unrecognized for a very long time. The human family of MYB transcription factors comprises MYB (c-MYB), MYBL2 (b-MYB), and MYBL1 (a-MYB), which are overexpressed in several cancers and are associated with cancer progression and resistance to anticancer drugs. In addition to overexpression, the presence of activated MYB-fusion proteins as tumor drivers was described in certain cancers. The identification of anticancer drug resistance mediated by MYB proteins and their underlying mechanisms are of great importance in understanding failures of current therapies and establishing new and more efficient therapy regimens. In addition, new drug candidates targeting MYB transcription factor activity and signaling have emerged as a promising class of potential anticancer therapeutics that could tackle MYB-dependent drug-resistant cancers in a more selective way. This review describes the correlation of MYB transcription factors with the formation and persistence of cancer resistance to various approved and investigational anticancer drugs.

C/EBPβ cooperates with MYB to maintain the oncogenic program of AML cells

Studies on the role of transcription factor MYB in acute myeloid leukemia (AML) have identified MYB as a key regulator of a transcriptional program for self-renewal of AML cells. Recent work summarized here has now highlighted the CCAAT-box/enhancer binding protein beta (C/EBPβ) as an essential factor and potential therapeutic target that cooperates with MYB and coactivator p300 in the maintenance of the leukemic cells.

MYB-NFIB fusion transcript in Adenoid Cystic Carcinoma: current state of knowledge and future directions

Adenoid cystic carcinoma (ACC) is the most common type of salivary gland cancer that can also arise in other primary sites. Regardless of the site, most ACC cases carry a recurrent chromosomal translocation - t(6;9)(q22-23;p23-24) - involving the MYB oncogene and the NFIB transcription factor. Generally, a long sequence of MYB is fused to the terminal exons of NFIB, yet the break can occur in different exons for both genes, resulting in multiple chimeric variants. The fusion status can be determined by a number of methods, each of them with particular advantages. In vitro and in vivo studies have been conducted to understand the biological consequences of MYB-NFIB translocation, and such findings could contribute to improving the current inefficient therapeutic options for disseminated ACC. This review provides a discussion on relevant evidence in the context of ACC MYB-NFIB translocations to determine the current state of knowledge and discuss future directions.

Transcription Factors as the "Blitzkrieg" of Plant Defense: A Pragmatic View of Nitric Oxide's Role in Gene Regulation

Plants are in continuous conflict with the environmental constraints and their sessile nature demands a fine-tuned, well-designed defense mechanism that can cope with a multitude of biotic and abiotic assaults. Therefore, plants have developed innate immunity, R-gene-mediated resistance, and systemic acquired resistance to ensure their survival. Transcription factors (TFs) are among the most important genetic components for the regulation of gene expression and several other biological processes. They bind to specific sequences in the DNA called transcription factor binding sites (TFBSs) that are present in the regulatory regions of genes. Depending on the environmental conditions, TFs can either enhance or suppress transcriptional processes. In the last couple of decades, nitric oxide (NO) emerged as a crucial molecule for signaling and regulating biological processes. Here, we have overviewed the plant defense system, the role of TFs in mediating the defense response, and that how NO can manipulate transcriptional changes including direct post-translational modifications of TFs. We also propose that NO might regulate gene expression by regulating the recruitment of RNA polymerase during transcription.

New tricks from an old oncogene: gene fusion and copy number alterations of MYB in human cancer

MYB is a leucine zipper transcription factor that is essential for hematopoesis and for renewal of colonic crypts. There is also ample evidence showing that MYB is leukemogenic in several animal species. However, it was not until recently that clear evidence was presented showing that MYB actually is an oncogene rearranged in human cancer. In a recent study, a novel mechanism of activation of MYB involving gene fusion was identified in carcinomas of the breast and head and neck. A t(6;9) translocation was shown to generate fusions between MYB and the transcription factor gene NFIB. The fusions consistently result in loss of the 3'-end of MYB, including several highly conserved target sites for microRNAs that negatively regulate MYB expression. Deletion of these target sites may disrupt the repression of MYB, leading to overexpression of MYB-NFIB transcripts and protein and to transcriptional activation of critical MYB target genes associated with apoptosis, cell cycle control, cell growth/angiogenesis and cell adhesion. This study, together with previous and recent data showing rearrangements and copy number alterations of the MYB locus in T-cell leukemia and certain solid tumors, will be the main focus of this review.

Avian myeoloblastosis virus (AMV): only one side of the coin

For many years, scientists and suppliers have refered to AMV-RT as the reverse transcriptase produced by the Avian Myelobalstosis Virus. This manuscript briefly reviews the molecular basis for biological dependence of AMV for the envelope and RT proteins that are produced by its natural helper the Myeloblastosis Associated Virus (MAV). Because the wide use of the term <<AMV RT>> obscures scientific facts, it is worthwhile to clarify this issue for the scientific community, especially for younger scientists who might not be aware of the functional relationships that exist between these two viruses.

The C-MYB locus is involved in chromosomal translocation and genomic duplications in human T-cell acute leukemia (T-ALL), the translocation defining a new T-ALL subtype in very young children

The C-Myb transcription factor is essential for hematopoiesis, including in the T-cell lineage. The C-Myb locus is a common site of retroviral insertional mutagenesis, however no recurrent genomic involvement has been reported in human malignancies. Here, we identified 2 types of genomic alterations involving the C-MYB locus at 6q23 in human T-cell acute leukemia (T-ALL). First, we found a reciprocal translocation, t(6;7)(q23;q34), that juxtaposed the TCRB and C-MYB loci (n = 6 cases). Second, a genome-wide copy-number analysis by array-based comparative genomic hybridization (array-CGH) identified short somatic duplications that include C-MYB (MYB(dup), n = 13 cases of 84 T-ALL, 15%). Expression analysis, including allele-specific approaches, showed stronger C-MYB expression in the MYB-rearranged cases compared with other T-ALLs, and a dramatically skewed C-MYB allele expression in the TCRB-MYB cases, which suggests that a translocation-driven deregulated expression may overcome a cellular attempt to down-regulate C-MYB. Strikingly, profiling of the T-ALLs by clinical, genomic, and large-scale gene expression analyses shows that the TCRB-MYB translocation defines a new T-ALL subtype associated with a very young age for T-cell leukemia (median, 2.2 years) and with a proliferation/mitosis expression signature. By contrast, the MYB(dup) alteration was associated with the previously defined T-ALL subtypes.

Transformation by v-Myb

The v-myb oncogene of the avian myeloblastosis virus (AMV) is unique among known oncogenes in that it causes only acute leukemia in animals and transforms only hematopoietic cells in culture. AMV was discovered in the 1930s as a virus that caused a disease in chickens that is similar to acute myelogenous leukemia in humans (Hall et al., 1941). This avian retrovirus played an important role in the history of cancer research for two reasons. First, AMV was used to demonstrate that all oncogenic viruses did not contain a single cancer-causing principle. In particular, although both Rous sarcoma virus (RSV) and AMV could replicate in cultures of either embryonic fibroblasts or hematopoietic cells, RSV could transform only fibroblasts whereas AMV could transform only hematopoietic cells (Baluda, 1963; Durban and Boettiger, 1981a). Second, chickens infected with AMV develop remarkably high white counts and therefore their peripheral blood contains remarkably large quantities of viral particles (Beard, 1963). For this reason AMV was often used as a prototypic retrovirus in order to study viral assembly and later to produce large amounts of reverse transcriptase for both research and commercial purposes. Following the discovery of the v-src oncogene of RSV and the demonstration that it arose from the normal c-src proto-oncogene, a number of acute leukemia viruses were analysed by similar techniques and found to also contain viral oncogenes of cellular origin (Roussel et al., 1979). In the case of AMV, it was shown that almost the entire retroviral env gene had been replaced by a sequence of cellular origin (initially called mab or amv, but later renamed v-myb) (Duesberg et al., 1980; Souza et al., 1980). Remarkably, sequences contained in this myb oncogene were shared between AMV and the avian E26 leukemia virus, but were not contained in any other acutely transforming retroviruses. In addition, the E26 virus contained a second sequence of cellular origin (ets) that was unique. The E26 leukemia virus was first described in the 1960s and causes an acute erythroblastosis in chickens, more reminiscent of the disease caused by avian erythroblastosis virus (AEV) than by AMV (Ivanov et al., 1962).

Identification and mapping of a common proviral integration site Fli-1 in erythroleukemia cells induced by Friend murine leukemia virus

Friend murine leukemia virus (F-MuLV) induces erythroleukemia when inoculated into newborn BALB/c or NIH/Swiss mice. We have molecularly cloned F-MuLV host cell DNA junction fragments from an erythroleukemia cell line induced by F-MuLV to identify cellular genes involved in the leukemogenic process. One particular proviral integration site, Fli-1, is rearranged in 75% (9/12) of independently isolated erythroleukemia cell lines derived from either BALB/c or NIH/Swiss mice inoculated at birth with F-MuLV. Other hematopoietic neoplasms induced by F-MuLV, including myeloid (granulocytic) and lymphoid tumors, did not show rearrangements of the Fli-1 locus. Similarly, none of 35 erythroleukemia cell lines induced by the Friend virus complexes (FV-A and FV-P) was rearranged at the Fli-1 locus. In contrast, no rearrangements were detected at the Sfpi-1 locus, a preferred site of integration in either FV-P- or FV-A-induced leukemias. Using recombinant inbred mice, the Fli-1 locus was situated on mouse chromosome 9 close to the cellular protooncogene c-ets-1. DNA and RNA analysis suggests, however, that Fli-1 is different from ets-1. Thus, Fli-1 appears to define a distinct locus specifically involved in the induction of erythroid leukemias by F-MuLV.

DNA-binding activity associated with the v-myb oncogene product is not sufficient for transformation

The product of the v-myb oncogene of avian myeloblastosis virus is a nuclear protein with an associated DNA-binding activity. We demonstrated that the highly conserved amino-terminal domain of p48v-myb is required for its associated DNA-binding activity. This activity is not required for the nuclear localization of p48v-myb. Furthermore, the associated DNA-binding activity and nuclear localization of p48v-myb together are not sufficient for transformation.

Cancer genes generated by rare chromosomal rearrangements rather than activation of oncogenes

The 20 known transforming onc genes of retroviruses are defined by sequences that are transduced from cellular genes, termed proto-oncogenes or cellular oncogenes. Based on these sequences, viral onc genes have been postulated to be transduced cellular cancer genes and proto-onc genes have been postulated to be latent cancer genes that can be activated from within the cell to cause virus-negative tumors. The hypothesis is popular because it promises direct access to cellular cancer genes. However, the existence of latent cancer genes presents a paradox since such genes are clearly undesirable. The hypothesis predicts (i) that viral onc genes and proto-onc genes are isogenic, (ii) that expression of proto-onc genes induces tumors, (iii) that activated proto-onc genes transform diploid cells upon transfection, like viral onc genes, and (iv) that diploid tumors exist that differ from normal cells only in transcriptionally or mutationally activated proto-onc genes. As yet, none of these predictions is confirmed. Moreover, the probability of spontaneous transformation in vivo is at least 10(9) times lower than predicted from the mechanisms thought to activate proto-onc genes. Therefore the hypothesis, that proto-onc genes are latent cellular oncogenes, appears to be an overinterpretation of sequence homology to structural and functional homology with viral onc genes. Here it is proposed that only rare truncations and illegitimate recombinations that alter the germline configuration of cellular genes, generate viral and possibly cellular cancer genes. The clonal chromosome abnormalities that are consistently found in tumor cells are microscopic evidence for rearrangements that may generate cancer genes. The clonality indicates that the tumors are initiated with, and possibly by, these abnormalities as predicted by Boveri in 1914 (Zur Frage der Entstehung maligner Tumoren, Jena, Fischer).

Target sequences for cis-acting regulation within the dual promoter of the human c-myc gene

Recombinant plasmids of the human c-myc promoter-leader region and the bacterial chloramphenicol acetyltransferase (cat) gene were constructed. After transfection into different rodent and human cells, the 862-base-pair (bp) PvuII fragment carrying both c-myc promoters and 350 bp of the untranslated leader conferred 1/15 to 1/30 of the CAT activity mediated by the simian virus 40 promoter. The presence of additional sequences upstream of the PvuII fragment had an overall negative effect on c-myc promoter activity detectable by titration analysis with small amounts of transfected plasmid DNA. The analysis of numerous deletion constructs in the c-myc promoter-leader region as well as S1 mapping experiments demonstrated that the high CAT activity depended largely on the presence of the second promoter. By cotransfection of c-myc-cat constructs with plasmids carrying different parts of the c-myc promoter locus, targets for positively acting cellular factors were identified. Two positive regulatory elements were mapped within the 862-bp PvuII fragment. One was localized within the 248-bp PvuII-SmaI fragment -101 to -349 bp upstream of the first cap site and the other within the 142-pb XhoI-NaeI fragment of the first exon, comprising positions -95 to +47 relative to the second cap site. We conclude that the dual promotor of the human c-myc gene represents a strong eucaryotic promotor regulated by cooperation of positively and negatively acting cellular transcription factors.

Insertion and truncation of c-myb by murine leukemia virus in a myeloid cell line derived from cultures of normal hematopoietic cells

A retroviral insertion into the c-myb gene, which resulted in a 3' truncation, was found in an in vitro-derived myeloid cell line. The retroviral insertion occurred at precisely the same nucleotide at which another murine leukemia virus insertion occurred in an in vivo-induced myeloid leukemia. These findings suggest that comparable events may be required for the derivation of myeloid cell lines in vitro and for induction of myeloid leukemia in vivo.

Cancer genes: rare recombinants instead of activated oncogenes (a review)

The 20 known transforming (onc) genes of retroviruses are defined by sequences that are transduced from cellular genes termed protooncogenes or cellular oncogenes. Based on these sequences, viral onc genes have been postulated to be transduced cellular cancer genes, and proto-onc genes have been postulated to be latent cancer genes that can be activated from within the cell to cause virus-negative tumors. The hypothesis is popular because it promises direct access to cellular cancer genes. However, the existence of latent cancer genes presents a paradox, since such genes are clearly undesirable. The hypothesis predicts that viral onc genes and proto-onc genes are isogenic; that expression of proto-onc genes induces tumors; that activated proto-onc genes transform diploid cells upon transfection, like viral onc genes; and that diploid tumors exist. As yet, none of these predictions is confirmed. Instead: Structural comparisons between viral onc genes, essential retroviral genes, and proto-onc genes show that all viral onc genes are indeed new genes, rather than transduced cellular cancer genes. They are recombinants put together from truncated viral and truncated proto-onc genes. Proto-onc genes are frequently expressed in normal cells. To date, not one activated proto-onc gene has been isolated that transforms diploid cells. Above all, no diploid tumors with activated proto-onc genes have been found. Moreover, the probability of spontaneous transformation in vivo is at least 10(9) times lower than predicted from the mechanisms thought to activate proto-onc genes. Therefore, the hypothesis that proto-onc genes are latent cellular oncogenes appears to be an overinterpretation of sequence homology to structural and functional homology with viral onc genes. Here it is proposed that only rare truncations and illegitimate recombinations that alter the germ-line configuration of cellular genes generate viral and possibly cellular cancer genes. The clonal chromosome abnormalities that are consistently found in tumor cells are microscopic evidence for rearrangements that may generate cancer genes. The clonality indicates that the tumors are initiated with, and possibly by, these abnormalities, as predicted by Boveri in 1914.

Target sequences for cis-acting regulation within the dual promoter of the human c-myc gene

Recombinant plasmids of the human c-myc promoter-leader region and the bacterial chloramphenicol acetyltransferase (cat) gene were constructed. After transfection into different rodent and human cells, the 862-base-pair (bp) PvuII fragment carrying both c-myc promoters and 350 bp of the untranslated leader conferred 1/15 to 1/30 of the CAT activity mediated by the simian virus 40 promoter. The presence of additional sequences upstream of the PvuII fragment had an overall negative effect on c-myc promoter activity detectable by titration analysis with small amounts of transfected plasmid DNA. The analysis of numerous deletion constructs in the c-myc promoter-leader region as well as S1 mapping experiments demonstrated that the high CAT activity depended largely on the presence of the second promoter. By cotransfection of c-myc-cat constructs with plasmids carrying different parts of the c-myc promoter locus, targets for positively acting cellular factors were identified. Two positive regulatory elements were mapped within the 862-bp PvuII fragment. One was localized within the 248-bp PvuII-SmaI fragment -101 to -349 bp upstream of the first cap site and the other within the 142-pb XhoI-NaeI fragment of the first exon, comprising positions -95 to +47 relative to the second cap site. We conclude that the dual promotor of the human c-myc gene represents a strong eucaryotic promotor regulated by cooperation of positively and negatively acting cellular transcription factors.

env-encoded residues are not required for transformation by p48v-myb

The v-myb oncogene of avian myeloblastosis virus induces acute myeloblastic leukemia in chickens and transforms avian myeloid cells in vitro. The protein product of this oncogene, p48v-myb, is partially encoded by the retroviral gag and env genes. We demonstrated that the env-encoded carboxyl terminus of p48v-myb is not required for transformation. Our results showed, in addition, that a coding region of c-myb which is not essential for transformation was transduced by avian myeloblastosis virus.

Expression of genes in cloned murine cell lines that can be maintained in both interleukin 2- and interleukin 3-dependent growth states

Two cloned murine cell lines, FD.C/1 and 32Dcl-23 exhibit switching of lymphokine-dependent growth states. The bone marrow-derived FD.C/1 and 32Dcl-23 cell lines are normally grown in culture medium supplemented with interleukin 3 (IL3). The replacement of IL3 with interleukin 2 (IL2) in the medium results in an increase in IL2 receptor expression in FD.C/1 and 32Dcl-23 cells and the switching of cells to an IL2-dependent growth state. We have compared patterns of protein and phosphoprotein synthesis, as well as the expression of the c-abl, c-ras, c-myb, and c-fos oncogenes in these cell lines maintained in IL3- and IL2-dependent growth states. The synthesis of a series of proteins and phosphoproteins are identified with each of the lymphokine-dependent growth states. All of the oncogenes examined are expressed in both IL2- and IL3-dependent cells and are not altered by phenotypic changes in lymphokine growth dependence. The relationship of oncogene expression to intracellular pathways regulated by lymphokine-receptor interactions is considered.

Two modes of c-myb activation in virus-induced mouse myeloid tumors

Two modes of disruption of the protooncogene c-myb by viral insertional mutagenesis in mouse myeloid tumor cells are described. The first mode was found in six tumors in which a Moloney murine leukemia virus component had inserted in the same transcriptional orientation upstream of the 5'-most exon with v-myb homology (vE1). cDNA sequence data indicate the presence of a truncated c-myb mRNA that is initiated in the upstream 5' long terminal repeat of the integrated provirus and processed via a cryptic splice donor sequence in the gag region to the splice acceptor site in vE1 of the c-myb gene, thus removing the remaining downstream viral and myb intronic sequences. Unlike most gag-onc transcripts, the gag and myb sequences in the hybrid transcript were not in the same reading frame. It is presumed that the gag sequence provides a cryptic translation initiation site for the novel amino-truncated c-myb protein. The second mode of disruption was by downstream virus insertion at the 3' side of the c-myb, which results in the synthesis of a small (approximately 2 kilobase) myb transcript. The 5' long terminal repeat of the inserted provirus provides a TGA termination codon that results in the elimination of 240 normal c-myb amino acid residues from the carboxyl terminus of the tumor-specific myb protein. These results suggest that truncated myb proteins play a role in neoplastic transformation of myeloid cells.

Subnuclear localization of proteins encoded by the oncogene v-myb and its cellular homolog c-myb

The retroviral transforming gene v-myb encodes a 45,000-Mr nuclear transforming protein (p45v-myb). p45v-myb is a truncated and mutated version of a 75,000-Mr protein encoded by the chicken c-myb gene (p75c-myb). Like its viral counterpart, p75c-myb is located in the cell nucleus. As a first step in identifying nuclear targets involved in cellular transformation by v-myb and in c-myb function, we determined the subnuclear locations of p45v-myb and p75c-myb. Approximately 80 to 90% of the total p45v-myb and p75c-myb present in nuclei was released from nuclei at low salt concentrations, exhibited DNA-binding activity, and was attached to nucleoprotein particles when released from the nuclei after digestion with nuclease. A minor portion of approximately 10 to 20% of the total p45v-myb and p75c-myb remained tightly associated with the nuclei even in the presence of 2 M NaCl. These observations suggest that both proteins are associated with two nuclear substructures tentatively identified as the chromatin and the nuclear matrix. The function of myb proteins may therefore depend on interactions with several nuclear targets.

Human c-myb protooncogene: nucleotide sequence of cDNA and organization of the genomic locus

We have isolated cDNA clones of the human c-myb mRNA that contain approximately 3.4 kilobases of the approximately 3.8-kilobase mRNA sequence. Nucleotide sequence analysis shows that the c-myb mRNA contains an open reading frame of 1920 nucleotides, which could encode a 72-kDa protein. The cDNA nucleotide sequence and the predicted amino acid sequence of the c-myb protein are highly homologous to the corresponding chicken and mouse proteins. In particular, a region toward the NH2 terminus of the protein containing a 3-fold tandem repeat of 51 residues is evolutionarily conserved and is the only region of homology with the Drosophila c-myb protein. This region may represent a functionally important structure, most likely the DNA-binding domain. cDNA clones have been used to isolate genomic clones and to define a preliminary intron/exon organization of the c-myb gene. Identification of 5' and 3' coding and noncoding exons indicates that the human c-myb locus spans a 40-kilobase region.

Transformation-defective mutant of avian myeloblastosis virus that is temperature sensitive for production of transforming protein p45v-myb

We have characterized a mutant of avian myeloblastosis virus (strain GA907/7) that shows a reduced capacity to transform myelomonocytic cells at the nonpermissive temperature. Myeloblasts transformed by this mutant suffer a substantial decrease in the amount of the transforming protein p45v-myb when shifted from the permissive to the nonpermissive temperature. We presume that the 5- to 10-fold decrease in the amount of p45v-myb causes the loss of the transformed phenotype. The decrease is due to a reduction in the level of v-myb mRNA. Mutant GA907/7 thus provides genetic evidence that p45v-myb is the transforming protein of avian myeloblastosis virus and apparently represents an unusual defect in the production or stability of mRNA.

Expression of molecular clones of v-myb in avian and mammalian cells independently of transformation

We demonstrated that molecular clones of the v-myb oncogene of avian myeloblastosis virus (AMV) can direct the synthesis of p48v-myb both in avian and mammalian cells which are not targets for transformation by AMV. To accomplish this, we constructed dominantly selectable avian leukosis virus derivatives which efficiently coexpress the protein products of the Tn5 neo gene and the v-myb oncogene. The use of chemically transformed QT6 quail cells for proviral DNA transfection or retroviral infection, followed by G418 selection, allowed the generation of cell lines which continuously produce both undeleted infectious neo-myb viral stocks and p48v-myb. The presence of a simian virus 40 origin of replication in the proviral plasmids also permitted high-level transient expression of p48v-myb in simian COS cells without intervening cycles of potentially mutagenic retroviral replication. These experiments establish that the previously reported DNA sequence of v-myb does in fact encode p48v-myb, the transforming protein of AMV.

Antibodies to the evolutionarily conserved amino-terminal region of the v-myb-encoded protein detect the c-myb protein in widely divergent metazoan species

Antibodies directed against a bacterial fusion protein that contains the domain encoded by the highly evolutionarily conserved 5' one-third of the v-myb oncogene of avian myeloblastosis virus (AMV) detect the protein products of various members of the myb gene family. Immunoprecipitation or immunoblot analyses with these antibodies yielded the following information. First, the products of the v-myb oncogenes of AMV (p48v-myb) and of E26 virus (p135gag-myb-ets) contain this highly conserved amino acid sequence, as previously hypothesized. Second, p75c-myb, the product of the chicken c-myb protooncogene, also contains this protein domain. Third, these antibodies have identified the products of the human, murine, and Drosophila c-myb genes, which were all found to be nuclear proteins of Mr 75,000-80,000. The human c-myb protein product is present in immature cells of the erythroid, myeloid, and lymphoid lineages.

Conserved chromosomal positions of dual domains of the ets protooncogene in cats, mice, and humans

The mammalian protooncogene homologue of the avian v-ets sequence from the E26 retrovirus consists of two sequentially distinct domains located on different chromosomes. Using somatic cell hybrid panels, we have mapped the mammalian homologue of the 5' v-ets-domain to chromosome 11 (ETS1) in man, to chromosome 9 (Ets-1) in mouse, and to chromosome D1 (ETS1) in the domestic cat. The mammalian homologue of the 3' v-ets domain was similarly mapped to human chromosome 21 (ETS2), to mouse chromosome 16 (Ets-2), and to feline chromosome C2 (ETS2). Both protooncogenes fell in syntenic groups of homologous linked loci that were conserved among the three species. The occurrence of two distinct functional protooncogenes and their conservation of linkage positions in the three mammalian orders indicate that these two genes have been separate since before the evolutionary divergence of mammals.

Two modes of c-myb activation in virus-induced mouse myeloid tumors

Two modes of disruption of the protooncogene c-myb by viral insertional mutagenesis in mouse myeloid tumor cells are described. The first mode was found in six tumors in which a Moloney murine leukemia virus component had inserted in the same transcriptional orientation upstream of the 5'-most exon with v-myb homology (vE1). cDNA sequence data indicate the presence of a truncated c-myb mRNA that is initiated in the upstream 5' long terminal repeat of the integrated provirus and processed via a cryptic splice donor sequence in the gag region to the splice acceptor site in vE1 of the c-myb gene, thus removing the remaining downstream viral and myb intronic sequences. Unlike most gag-onc transcripts, the gag and myb sequences in the hybrid transcript were not in the same reading frame. It is presumed that the gag sequence provides a cryptic translation initiation site for the novel amino-truncated c-myb protein. The second mode of disruption was by downstream virus insertion at the 3' side of the c-myb, which results in the synthesis of a small (approximately 2 kilobase) myb transcript. The 5' long terminal repeat of the inserted provirus provides a TGA termination codon that results in the elimination of 240 normal c-myb amino acid residues from the carboxyl terminus of the tumor-specific myb protein. These results suggest that truncated myb proteins play a role in neoplastic transformation of myeloid cells.

Subnuclear localization of proteins encoded by the oncogene v-myb and its cellular homolog c-myb

The retroviral transforming gene v-myb encodes a 45,000-Mr nuclear transforming protein (p45v-myb). p45v-myb is a truncated and mutated version of a 75,000-Mr protein encoded by the chicken c-myb gene (p75c-myb). Like its viral counterpart, p75c-myb is located in the cell nucleus. As a first step in identifying nuclear targets involved in cellular transformation by v-myb and in c-myb function, we determined the subnuclear locations of p45v-myb and p75c-myb. Approximately 80 to 90% of the total p45v-myb and p75c-myb present in nuclei was released from nuclei at low salt concentrations, exhibited DNA-binding activity, and was attached to nucleoprotein particles when released from the nuclei after digestion with nuclease. A minor portion of approximately 10 to 20% of the total p45v-myb and p75c-myb remained tightly associated with the nuclei even in the presence of 2 M NaCl. These observations suggest that both proteins are associated with two nuclear substructures tentatively identified as the chromatin and the nuclear matrix. The function of myb proteins may therefore depend on interactions with several nuclear targets.

Transformation-defective mutant of avian myeloblastosis virus that is temperature sensitive for production of transforming protein p45v-myb

We have characterized a mutant of avian myeloblastosis virus (strain GA907/7) that shows a reduced capacity to transform myelomonocytic cells at the nonpermissive temperature. Myeloblasts transformed by this mutant suffer a substantial decrease in the amount of the transforming protein p45v-myb when shifted from the permissive to the nonpermissive temperature. We presume that the 5- to 10-fold decrease in the amount of p45v-myb causes the loss of the transformed phenotype. The decrease is due to a reduction in the level of v-myb mRNA. Mutant GA907/7 thus provides genetic evidence that p45v-myb is the transforming protein of avian myeloblastosis virus and apparently represents an unusual defect in the production or stability of mRNA.

Developmental regulation of c-myb in normal myeloid progenitor cells

Hematopoietic tissues and some leukemic cell lines express elevated levels of c-myb transcripts. We have separated a subpopulation of chicken embryo yolk sac cells that represents about 5% of the yolk sac hematopoietic cells and appears to contain all of the detectable c-myb transcripts. The level of myb expression in this cell population is higher than previously reported for any normal cell population and is in the range of that found in cells transformed by avian myeloblastosis virus and E26 virus. Since the myb gene probe used also detects full-length viral transcripts as well as the v-myb mRNA, it appears that the level of expression of c-myb in this normal population may exceed that found in some transformed cell populations that depend on v-myb to maintain the transformed phenotype. This c-myb-expressing cell population has been identified as primarily M-CFC, the committed progenitor for the macrophage lineage. As cells differentiate to the promonocyte stage there is an abrupt decrease in c-myb expression of greater than 100 fold. These studies thus describe a normal cell population that expresses c-myb at levels similar to the level of v-myb in cells that depend on v-myb for the maintenance of their transformed phenotype. Furthermore, these studies provide direct evidence for the developmental regulation of c-myb during the process of normal macrophage differentiation.

Transformation of Brown Leghorn chicken embryo fibroblasts by avian myeloblastosis virus proviral DNA

Brown Leghorn chicken embryo fibroblasts were transfected with a mixture of avian myeloblastosis virus (AMV) and myeloblastosis-associated virus type 1 (MAV1) proviral DNA purified from lambda-Charon 4A recombinant clones. A transformed cell line (T1AM) able to grow without anchorage in semisolid medium was obtained. The presence of both proviral AMV and MAV sequences was detected in T1AM DNA by hybridization with v-myb- and MAV1-specific probes. Altered AMV and MAV1 proviral genomes were found in T1AM genome. Characterization of the RNA species expressed in transformed cells showed that in addition to a 2.5-kilobase (kb) putative subgenomic v-myb-specific RNA, three other myb-containing RNAs (9.4, 8.4, and 7.0 kb) were present in T1AM cells. No AMV genomic RNA was detected. Also, a new 5.0-kb MAV1-specific RNA species was expressed in transformed cells in addition to MAV1 genomic RNA species (7.8 kb). No infectious AMV virions are released by T1AM cells. Chicken embryo fibroblasts infected by T1AM-released virions contained and expressed all MAV1 sequences detected in T1AM transformed cells but did not express any transformation parameter. These results indicated that the presence of AMV proviral sequences in T1AM cells is responsible for their transformed phenotype.

The location of v-src in a retrovirus vector determines whether the virus is toxic or transforming

We prepared infectious stocks of an avian retrovirus, a modified spleen necrosis virus, containing the herpes simplex virus type 1 thymidine kinase gene and the avian sarcoma virus v-src gene. Viruses were recovered after cotransfection of chicken cells with DNA of recombinants between cloned spleen necrosis virus thymidine kinase and v-src and with DNA of cloned reticuloendotheliosis virus strain A. When v-src was inserted near the 5'end of the viral genome, only low titers of recombinant virus were recovered. Most of the recovered viruses were smaller than expected and did not transform the morphology of rat or chicken cells. A very small amount of virus of the expected structure was recovered; this virus transformed rat cells and expressed v-src. Cotransfection data indicated that one reason we failed to recover a significant titer of recombinant virus is that efficient expression of v-src is acutely toxic to chicken and dog cells. Insertion of v-src near the 3' end of the viral genome, such that it was expressed at a lower level compared with the 5'-v-src-containing virus, yielded a higher titer of recombinant virus, and this virus was transforming. The differences in the recovery and transforming activity of these viruses indicate that the location of an oncogene in the viral genome is an important factor regulating the level of its expression and whether or not this expression is toxic or transforming to cells.

The location of v-src in a retrovirus vector determines whether the virus is toxic or transforming

We prepared infectious stocks of an avian retrovirus, a modified spleen necrosis virus, containing the herpes simplex virus type 1 thymidine kinase gene and the avian sarcoma virus v-src gene. Viruses were recovered after cotransfection of chicken cells with DNA of recombinants between cloned spleen necrosis virus thymidine kinase and v-src and with DNA of cloned reticuloendotheliosis virus strain A. When v-src was inserted near the 5'end of the viral genome, only low titers of recombinant virus were recovered. Most of the recovered viruses were smaller than expected and did not transform the morphology of rat or chicken cells. A very small amount of virus of the expected structure was recovered; this virus transformed rat cells and expressed v-src. Cotransfection data indicated that one reason we failed to recover a significant titer of recombinant virus is that efficient expression of v-src is acutely toxic to chicken and dog cells. Insertion of v-src near the 3' end of the viral genome, such that it was expressed at a lower level compared with the 5'-v-src-containing virus, yielded a higher titer of recombinant virus, and this virus was transforming. The differences in the recovery and transforming activity of these viruses indicate that the location of an oncogene in the viral genome is an important factor regulating the level of its expression and whether or not this expression is toxic or transforming to cells.

Avian myeloblastosis virus and E26 virus oncogene products are nuclear proteins

The defective acute leukemia viruses avian myeloblastosis virus (AMV) and E26 virus each contain an inserted cellular sequence related to the same highly conserved cellular gene, proto-amv. The oncogenes of these two retroviruses differ from this cellular proto-oncogene in gene structure, transcript structure, and gene product. The product of the AMV oncogene (myb) is a 48,000 Mr protein, p48myb, encoded by a transduced segment (amv) of proto-amv flanked by short helper-virus-derived terminal sequences. The E26 virus oncogene product is a 135,000 Mr protein, p135gag-amve-ets, encoded by significant portions of a viral structural gene (gag), sequences related to proto-amv (amve), and additional E26-specific sequences (ets) transduced from cellular proto-ets. Both p48myb and p135gag-amve-ets transforming proteins are located in the nucleus of cells transformed by these viruses. A protein of 110,000 Mr which is specifically immunoprecipitated by antisera to amv peptides and may be the product of the normal cellular gene (proto-amv) has been located in the cytoplasm of cells that express proto-amv mRNA.

A relationship between the yeast cell cycle genes CDC4 and CDC36 and the ets sequence of oncogenic virus E26

We report here significant primary sequence homology among the predicted translational products of three genes: CDC4 , CDC36 and ets. CDC4 and CDC36 are Saccharomyces cerevisiae cell division cycle genes, while ets is a transformation-specific sequence of avian erythroblastosis virus E26. The deduced primary structures of the three gene products were compared by computer to a large data base of known and predicted protein sequences. The search revealed 22.0-25.5% identity over regions of 140-206 codons, respectively between the different pairwise combinations. For these particular sequences, these identity scores fall 3.4-4.0 standard deviations above the empirically-determined mean values of fortuitous similarity. S. cerevisiae calls require CDC36 and CDC4 in order to complete two early events in the cell cycle: execution of start ( CDC36 ) and spindle pole body separation ( CDC4 ). In virus E26, the ets sequence is linked in frame with delta gag and mybE in the tripartite structure 5'-delta gag- mybE -ets-3', comprising the E26 transforming oncogene. The homologies described here suggest that the biochemical functions or regulation of the CDC4 , CDC36 and ets products may be related.

Transduction of c-myb into avian myeloblastosis virus: locating points of recombination within the cellular gene

The oncogene (v-myb) of avian myeloblastosis virus apparently arose by transduction of nucleotide sequences from a cellular gene (c-myb). In c-myb the nucleotide sequences that formed v-myb exist at seven distinct regions separated by nontransduced stretches of sequence that are flanked by eucaryotic splice signals. By contrast, the sequences at the outside boundaries of the transduced region of c-myb do not resemble splice sites. We mapped the nucleotide sequences that are homologous to the ends of v-myb with respect to the exons and introns of c-myb. The results indicate that the leftward recombination between c-myb and the transducing retrovirus occurred within an intron of the cellular gene, whereas the rightward recombination took place in an exon of c-myb. Transduction of c-myb sequences may therefore have involved a DNA rearrangement.

Tripartite structure of the avian erythroblastosis virus E26 transforming gene

Only two avian oncogenic viruses specifically cause acute leukaemias yet do not transform chicken fibroblasts in culture: E26, which causes erythroblastosis and a low level of concomitant myeloblastosis in chickens, and avian myeloblastosis virus (AMV), which causes myeloblastosis exclusively. Both viruses are replication-defective and share a sequence termed myb (also known as amv) which is unrelated to essential virion genes and is therefore thought to be part of the transforming onc genes of these viruses. However, the genetic structure of the two viruses differs. E26 has a genomic RNA of 5.7 kilobases (kb) and encodes a 135,000 molecular weight gag-related protein (p135) with probable transforming function. We show here by in vitro translation that the 5.7-kb E26 RNA directs the synthesis of p135. Oligonucleotide analysis indicates that E26 RNA contains an internal 0.8-kb subset of the 1.2-kb AMV-related sequence (mybA), termed mybE. A 2.46-kb molecular clone prepared from cDNA transcribed in vitro from E26 RNA contained an E26 transformation-specific (ets) sequence flanked by mybE and an env-related sequence. A complete DNA sequence of this clone indicates that the 1.5-kb ets sequence extends the open reading frame of mybE for 491 amino acids. Thus, the p135 gene of E26 is a genetic hybrid of three distinct elements, approximately 1.2 kb derived from the 5' region of the retroviral gag gene, mybE and the ets sequence, linked in the order 5'-delta gag-mybE-ets-3'. The myeloid leukaemogenicity shared by E26 and AMV correlates with the common myb sequence, while the distinct erythroid leukaemogenicity of E26 correlates with ets and the E26-specific linkage of myb to delta gag.

Retroviral transforming genes in normal cells?

The hallmark of retroviral transforming genes (onc genes) are specific sequences which are unrelated to essential virion genes but are closely related to sequences in normal cells. Viral onc genes probably originated from rare transductions of these cellular sequences by retroviruses without onc genes. Consequently, it has been suggested that retroviral transforming genes are present in normal cells in a latent form. However, recent structural analyses indicate that viral onc genes and cellular genes, which share specific sequences, are not isogenic. They differ from each other in scattered point mutations and in unique coding regions. The cellular genes containing onc-related sequences are expressed in normal cells compatible with a normal function. There is as yet no functional or consistent circumstantial evidence that these cellular genes cause cancer in animals that are not infected by viruses with onc genes. Therefore, it is still uncertain whether the onc-related cellular genes have oncogenic potential beyond their role as progenitors of retroviral onc genes.

Identification of the leukemogenic protein of avian myeloblastosis virus and of its normal cellular homologue

The genome of the replication-defective avian myeloblastosis virus (AMV) contains an inserted cellular sequence (amv) that is part of the oncogene responsible for acute myeloblastic leukemia in chickens infected with AMV. Three antisera raised against distinct synthetic peptides predicted from the long open reading frame of amv specifically precipitated the same 48-kilodalton protein (p48amv) from leukemic myeloblasts but not from normal hematopoietic tissue, fibroblasts, or from fibroblasts infected with the AMV helper virus, MAV-1 (myeloblastosis-associated virus type 1). p48amv is not glycosylated or phosphorylated and does not appear to act as a protein kinase in vitro. The same three antisera that recognized p48amv also specifically precipitated a common 110-kilodalton protein from normal uninfected hematopoietic tissue. This normal cellular homologue of the AMV leukemogenic protein, p110proto-amv, was not present in normal fibroblasts, MAV-1 infected fibroblasts, or, interestingly, in some leukemic myeloblasts. We conclude that p48amv is the leukemogenic product of an altered, transduced, partial protooncogene. Short helper-virus sequences provide its carboxyl terminus and also may provide the amino terminus.

Structure and transcription of the cellular homolog (c-myb) of the avian myeloblastosis virus transforming gene (v-myb)

We isolated and characterized molecular clones containing the chicken cellular homolog (c-myb) of the avian myeloblastosis virus oncogene (v-myb). Mapping of the c-myb clones using restriction endonucleases and hybridization to radiolabeled v-myb probes revealed that the sequences homologous to v-myb are contained within four separate regions, which have since been shown by nucleotide sequencing (Klempnauer et al., Cell 31:453-463, 1982) to carry seven exons. Analysis of c-myb transcripts showed the presence of several large precursor RNAs in addition to the 4.0-kilobase cytoplasmic mRNA. We also determined the approximate positions of the c-myb sequences that are present in the 4.0-kilobase c-myb mRNA but not present in v-myb. Some of these sequences are found in a separate region 5' to the v-myb-related sequences, whereas the remainder appear to be located immediately 3' to the v-myb-related sequences. The data presented here, in conjunction with nucleotide sequence analysis (Klempnauer et al., Cell 31:453-463, 1982), indicate that the c-myb gene contains at least eight exons which span a total of about 16 kilobase pairs. The presence of exon sequences in c-myb outside the regions of homology with v-myb raises the possibility that the v-myb and c-myb gene products may differ significantly.

Isolation and characterization of a temperature-sensitive mutant of avian myeloblastosis virus

A temperature-sensitive (ts) mutant, GA 907/7, was isolated after mutagen treatment of avian myeloblastosis virus. When bone marrow cells or secondary yolk sac macrophages were infected with GA 907/7, the expression of transformation was greatly reduced at 41 degrees C. The results of temperature-shift experiments suggest that in GA 907/7 the putative v-myb gene product is functional only at 35.5 degrees C. Moreover, when ts-induced transformed cells were shifted to 41 degrees C, a partial morphological conversion to macrophage-like cells was obtained, while the majority of the cells underwent senescence and lysis. No leukemia was obtained when GA 907/7 was injected in 1-day-old chickens. Finally, a continuous cell line releasing genetically stable mutant virus was obtained after transformation of secondary yolk sac cells.

Comparison of the oligosaccharide moieties of the major envelope glycoproteins of the subgroup A and subgroup B avian myeloblastosis-associated viruses

The nature of the oligosaccharide chains of the major envelope glycoprotein, gp85, from avian myeloblastosis-associated viruses has been examined for the subgroup A and subgroup B viruses replicated in fibroblasts from the same chicken embryos. Pronase-digested glycopeptides from [3H]mannose- or [3H]glucosamine-labeled viruses were analyzed by the combined techniques of gel filtration, endo-beta-N-acetylglucosaminidase digestion, and concanavalin A affinity chromatography. The gp85 protein from these two viruses, and also from another subgroup A avian leukosis virus replicated in the same cells, contained a diverse array of asparagine-linked oligosaccharides of the acidic type [(sialic acid +/- galactose-N-acetylglucosamine)2-4-(mannose)3-N-acetylglucosamine2(+/- fucose)-asparagine], hybrid type (sialic acid +/- galactose-N-acetylglucosamine-(mannose)5,4-N-acetylglucosamine2-asparagine), and neutral type [(mannose)5-9-N-acetylglucosamine2-asparagine], with the more highly branched (tri or tetraantennary or both) acidic-type structures representing the predominant class of oligosaccharide. Minor differences were observed between the gp85 of the subgroup B versus subgroup A viruses.

The transforming gene of avian myeloblastosis virus (AMV): nucleotide sequence analysis and identification of its translational product

The genome of the avian myeloblastosis virus (AMV) has undergone a sequence substitution in which a portion of the region normally coding for the env protein has been replaced by cellular sequences. We have determined the complete nucleotide sequence of this region. Examination of the AMV oncogenic sequence revealed an open reading frame starting with the initiation codon ATG and terminating with the triplet TAG within the acquired cellular sequences and terminating with the triplet TAG at a point thirty-three nucleotides into helper viral sequences to the right of the helper-viral-cellular junction. The stretch of 795 nucleotides would code for a protein of 265 amino acids with a molecular weight of 30,000 daltons. The eleven amino acids at the carboxy terminus of such a protein would be derived from the env gene of helper virus. Antibodies were prepared against synthetic peptides derived from the predicted amino acid sequences. One such antibody precipitated two magnesium proteins of apparent nucleotide weight of 30,000 daltons and 51,000 daltons.

Acute leukemia viruses E26 and avian myeloblastosis virus have related transformation-specific RNA sequences but different genetic structures, gene products, and oncogenic properties

Replication-defective acute leukemia viruses E26 and myeloblastosis virus (AMV) cause distinct leukemias although they belong to the same subgroup of oncogenic avian tumor viruses based on shared transformation-specific (onc) RNA sequences. E26 causes predominantly erythroblastosis in chicken and in quail, whereas AMV induces a myeloid leukemia. However, upon cultivation in vitro for >1 month, a majority of surviving hemopoietic cells of E26-infected animals bear myeloid markers similar to those of AMV-transformed cells. We have analyzed the genetic structure and gene products of E26 virus for a comparison with those of AMV. An E26/helper virus complex was found to contain two RNA species: a 5.7-kilobase (kb) RNA that hybridizes with cloned AMV-specific proviral DNA and hence is probably the E26 genome; and an 8.5-kb RNA that is unrelated to AMV and represents helper virus RNA. Thus, E26 RNA is smaller than 7.5-kb AMV RNA. Hybridization of size-selected poly(A)-terminating E26 RNA fragments with AMV-specific DNA indicated that the shared specific sequences are located in the 5' half of the E26 genome as opposed to a 3' location in AMV RNA. In nonproducer cells transformed in vitro by E26, a gag-related nonstructural 135,000-dalton protein (p135) was found. No gag(Pr76) or gag-pol (Pr180) precursors of essential virion proteins, which are present in AMV nonproducer cells, were observed. p135 was also found in cultured E26 virus producing cells of several leukemic chickens, and its intracellular concentration relative to that of the essential virion proteins encoded by the helper virus correlates with the ratio of E26 to helper RNA in virions released by these cells. p135 is phosphorylated but not glycosylated; antigenically it is not related to the pol or env gene products. It appears to be coded for by a partial gag gene and by E26-specific RNA sequences, presumably including those shared with AMV. Hence, AMV and E26 appear to use different strategies for the expression of related onc sequences: AMV is thought to encode a transforming protein via a subgenomic mRNA, whereas E26 codes for a gag-related polyprotein via genomic RNA. It is speculated that differences in the oncogenic properties of E26 and AMV are due to differences in their genetic structures and gene products.

Nucleotide sequence analysis of the long terminal repeat of avian myeloblastosis virus and adjacent host sequences

The nucleotide sequence of the integrated avian myeloblastosis virus long terminal repeat has been determined. The sequence is 385 base pairs long and is present at both ends of the viral DNA. The cell-virus junctions at each end consist of a 6-base-pair direct repeat of cell DNA next to the inverted repeat of viral DNA. The long terminal repeat also contains promoter-like sequences, an mRNA capping site, and polyadenylation signals. Several features of this long terminal repeat suggest a structural and functional similarity with sequences of transposable and other genetic elements. Comparison of these sequences with long terminal repeats of other avian retroviruses indicates that there is a great variation in the 3' unique sequence (U3), whereas the 5' specific sequences (U5) and the R region are highly conserved.